The phrase "great deception" refers to a situation where someone or something tricks or misleads others on a large scale. It represents a significant and elaborate lie or deceitful act that can deceive a large number of people. Full definition
The Canadian climatologist, who has a Ph.D. in climatology from the University of London and taught at the University of Winnipeg for 28 years says that the widely propagated «fact» that humans are contributing to global warming is the «greatest deception in the history of science.» (desmogblog.com)
«32 Those who pretend to be trying to change their behavior while pleading with God for forgiveness yet go right back to the muddy wallow are opening themselves up to an even greater deception — the mocking of God's pardon. (religion-online.org)